Applied in industrial measuring- and automation technology, especially also in connection with the automation of chemical processes or procedures for producing a product from a raw or starting material by use of chemical, physical or biological processes and/or the automated control of industrial plants, directly installed at the respective plant, at times, also referred to as field devices, are measuring devices, such as e.g. Coriolis mass flow measuring devices, and density measuring devices, magneto inductive flow measuring devices, vortex flow measuring devices, ultrasound flow measuring devices, thermal mass flow measuring devices, pressure measuring devices, fill level measuring devices, temperature measuring devices, pH measuring devices, etc., which serve for ascertaining measured values representing a time variable, physical and/or chemical measured variable as well as producing, in each case, at least one digital or analog measured value signal for transmitting the measured values externally of the respective measuring device. The measured variable to be registered by means of the respective measuring device can, depending on application, be, for example, a mass flow, a density, a viscosity, a fill level- or a limit level, a pressure, a pH value, an electrical conductivity or a temperature or the like, of a liquid, powdered-, vaporous- or gaseous medium, which is conveyed, respectively held, in a corresponding container, such as e.g. a pipeline or a tank. Such measuring devices, known, per se, to those skilled in the art, are disclosed in, among others, European patent, EPA 1 591 977, British patent, GB A 2,229,897, USA 2001/0016802, USA 2010/0026322, USA 2011/0062942, U.S. Pat. Nos. 5,672,975 A, 6,014,100 A, 6,140,940 A, 6,452,493 B, 6,472,884 B, 6,684,340 B, 7,162,651 B, 7,296,482 B, 7,630,844 B, US 7,778,784 B, 7,792,646 B, Published International Application, WO A 00/26739, WO A 00/48157, WO A 01/71291, WO A 03/106931, WO A 2008/091548, WO A 2009/002341, WO A 2011/005938, WO A 2012/009003, WO A 2012/159683, WO A 2012/163608, the WO A 88/02476, WO A 88/02853, WO A 94/20940, WO A 95/08123, WO A 95/08758 or also the not pre-published international application PCT/EP2013/074632 of the applicant, respectively are sold industrially by Endress+Hauser Group of Companies, for example, under the designations, t trend ATT12, Flowphant T DTT31, Promag 53H, Prowirl 73F, Promass E 200, Promass F 200, Promass 83X, or Promass 84F.
Measuring devices of the above indicated type are typically formed by associating a physical-electrical or chemical-electrical measuring transducer serving for registering the respective process variable with a measurement transmitter electrically coupled therewith. Said measuring transducer is most often embodied to be applied in a wall of the container containing the medium or in the course of a line, for example, a pipeline, conveying the medium, and to generate at least one, firstly, analog, electrical transducer signal corresponding with the measured variable to be registered, namely representing its value as a function of time. The electrical transducer signal can, in turn, be further processed by means of a transmitter electronics of the respective measurement transmitter electrically connected with the measuring transducer, in such a manner that during operation of the measuring device corresponding measured values for the measured variable are ascertained by means of the transmitter electronics. The transmitter electronics of an industrial grade measuring device is most often accommodated in a comparatively robust, for instance, impact-, pressure-, explosion- and/or weather resistant, transmitter housing. Such housing can be arranged e.g. removed from the measuring transducer and connected with such only via a flexible cable; the housing can, however, also be arranged directly on the measuring transducer, respectively on a measuring transducer housing separately housing the measuring transducer.
For processing the transducer signal, especially namely for producing digital, measured values representing the particular measured variable and for transforming such measured values into at least one measured value signal transferable- and evaluatable externally of the measurement transmitter, respectively of the measuring device formed therewith, the transmitter electronics includes, furthermore, a corresponding measuring circuit. As also disclosed in, among others, the above mentioned US-A 2001/0016802, US-A 2010/0026322, US-A 2011/0062942, U.S. Pat. Nos. 7,630,844 B, 7,792,646 B, 7,778,784 B, 6,452,493 B, 6,014,100 A, WO-A 95/08123 or WO-A 2012/009003, the measuring circuits in the case of modern measuring devices of the type being discussed are most often formed by means of a microprocessor, which is fed the transducer signal (for example, a transducer signal already digitized earlier by means of a separate analog to digital converter) via a corresponding signal input. The microprocessor is, among other things, also provided to generate, based on the (in given cases, earlier digitized) transducer signal on the signal input, a measured value sequence, namely a sequence of digital, measured values instantaneously representing the measured variable at different points in time. The measured values generated by means of the measuring circuit are provided in the form of electrical, digital or analog, measured value signals, for example, in the form of an analog signal current correspondingly modulated in the range from 4-20 mA, on circuit outputs located equally within the transmitter housing and functioning, in each case, as an interface. Moreover, usual for making measured values available are also so-called frequency outputs, namely circuit outputs coding the measured values in a pulse sequence frequency of a binary rectangular signal, or also so-called pulse outputs, namely circuit outputs signaling in the form of a pulse the reaching of an earlier selectable, quantized, quantity unit.
Measuring devices of the type being discussed can supplementally or alternatively to circuit outputs providing measured value signals also have circuit outputs—, at times, also referred to as switch- or relay output-circuit outputs, which are adapted selectively to switch in—, respectively to switch out, an electrical circuit extending partially externally of the measuring device and, at times, also referenced as a load current loop, respectively to output a signal, most often a binary signal, for instance, in the form of a status signal for reporting an operating state of the respective measuring device and/or for transmission of an error report, respectively an alarm, to a remote reporting system or in the form of a switch signal serving for signaling a reaching of a predetermined threshold value and/or for the immediate actuating of external electrical devices connected to the measuring device, for example, an additional measuring device or, however, also a valve, a pump or a motor. Such circuit outputs can be formed, for example, by means of an electronic semiconductor switching element, such as e.g. a transistor or a thyristor, respectively be embodied as a semiconductor relay (solid state relay); widely used still, however, are also circuit outputs, which are formed by means of an electromechanical relay, thus an electromechanical switch, located within the transmitter housing and electrically connected with the transmitter electronics, respectively operated thereby.
Such electromechanical switches, respectively relays, installed in circuit outputs of measurement transmitters comprise, in each case, a coil arrangement formed by means of at least one electrical coil and a first switch contact movable therewith, consequently mechanically actuated between a first end position and a second end position, and a second switch contact. Said first switch contact, for the purpose of implementing two different switch positions, is adapted to be spaced in the first switch position from the second switch contact to form a high-resistance, respectively electrically insulating, air path, and in the second switch position to contact the second switch contact to form a low-ohm, respectively electrically conductive, connection. The coil of the coil arrangement includes a coil core and an electrical current conductor surrounding the coil core. Furthermore, a magnetic circuit is formed by means of the coil core as well as an armature of the coil arrangement, for example, a permanently magnetic armature and/or a metal armature, movable between a first end position and a second end position, wherein the coil is provided to lead an electrical current in the electrical current conductor and to convert the electrical current into a magnetic force acting between the coil core and the armature for moving the armature, and wherein the armature is provided, in the case of moving from its first end position into its second end position, to move the first switch contact from its first switch position into its second switch position, consequently to actuate the electromechanical switch.
Coil arrangements of the above indicated type, especially coil arrangements namely embodied as a component of a circuit output of a measurement transmitter, comprise, furthermore, an electrically controlled, electronic switch, for example, an electronic switch formed by means of at least one electronic, semiconductor element, such as, for instance, a transistor, switchable between at least two switch states as well as a control circuit, for example, also in combination with the previously indicated measuring circuit, namely also formed by means of its, in given cases, present microprocessor, for producing a switching signal with a variable signal level actuating the electronic switch, in such a manner that the switching signal has at times a first signal level value (“H”) causing a first switch state of the electronic switch, respectively the switching signal has at times a second signal level value (“L”) different from the first signal level value and causing a second switch state of the electronic switch. The electronic switch is, in turn, provided, in its first switch state, to close a coil electrical current circuit formed therewith and by the coil, in such a manner that an electrical resistance of the electrical current circuit opposing an electrical current then flowing in operation in the coil electrical current circuit then has a low-ohm, first resistance value, and in its second switch state to open the coil electrical current circuit, in such a manner that its electrical resistance then has a high-ohm, second resistance value, which is greater than the first resistance value. The measuring circuit is, furthermore, provided to generate a control signal instructing the control circuit, namely causing a change of the signal level of the switching signal from the first signal level to the second signal level or a change of the signal level of the switching signal from the second signal level to the first signal level, for example, also with application of the transducer signal and/or as a function of at least one of the measured values.
Advantages of such a circuit output formed by means of an electromechanical relay, respectively switch, include besides the relatively cost effective construction, among other things, that, structurally related, there can be implemented, on the one hand, a galvanic isolation, along with a high isolating resistance, respectively a high dielectric strength, of the so formed isolating distance between the control circuit, respectively the transmitter electronics (control current loop) formed therewith, and the electrical circuit (load current loop) extending externally of the measuring device and switched by means of the two switch contacts, and, on the other hand, a switch with very high-ohm contact junction resistance in the opened, first switch position, respectively very low-ohm contact junction resistance in the closed, second switch position, consequently with good distinguishability of the two switch states. Moreover, by means of an electromechanical relay in the load current loop, an electrical power very high in comparison to the electrical power converted in the control circuit, consequently in the control current loop, can be switched, respectively a circuit output with high overloadability and/or high short-circuit strength can be provided. Electromechanical relays, respectively circuit outputs formed therewith, exhibit regularly a high electromagnetic compatibility (EMC) and high disturbance resistance, even against electrostatic discharge (ESD).
A disadvantage of an electromechanical relay, respectively a switch formed therewith, is that its movable components, namely the armature and the switch contacts, even in the case of loading according to specification, wear mechanically, consequently such electromechanical switches can undergo, in comparison to semiconductor relays, a significantly smaller number of switching cycles, respectively have, in comparison to semiconductor relays, nominally a comparatively lower maximum number of switching cycles. Moreover, the, at the start, very low ohm contact junction resistance of such a switch in its closed (second) switch position can change as a function of the electrical powers switched during operation, in such a manner that the contact junction resistance increases with increasing duration of operation, respectively increasing number of switching cycles, in an undesired manner. Furthermore, shaking, respectively vibration, of the transmitter electronics negatively affects the ability of the respective electromechanical switch, respectively the circuit output formed therewith, to function.
Unfavorable constellations of such loadings regularly not detectable by the transmitter electronics or, in given cases, overloadings of electromechanical relay, respectively a circuit output formed therewith, equally not detected, also exceeding earlier specified limit values, can, in given cases, even lead to a total failure of the relay, for example, namely in such a manner that the armature of the relay—at times, or continually—, in spite of an alternation of the control signal from one signal level value to another, remains in an assumed end position, for example, as a result of a jamming or seizing of the armature and switch contact actuated therewith, because of deformation or breaking off of the armature or as a result of a welding or sticking together of the two switch contacts. As a result of this, the electrical circuit corresponding to the load current loop of the relay and extended externally of the measuring device can no longer be switched by means of the respective circuit output, respectively a signal can no longer be output via the circuit output. Particularly in safety related applications, such a total failure of the circuit output can lead to fatal consequences for the affected plant, respectively its environment, and must, consequently, be detected, respectively eliminated, as soon as possible.